1. Field of the Invention
The present invention relates to optical beam scanning devices for use in a laser beam printer, a digital copying machine, etc., and in particular relates to an optical beam scanning device suitable for use in an over illumination scanning optical system in that the width of a beam in a main scanning direction incident on a polygon mirror is larger than a reflection surface of the polygon mirror in the main scanning direction.
2. Description of the Related Art
The optical beam scanning device is a generally known technique. In addition, a rotational axial direction of a deflector is referred to as a subscanning direction and a direction perpendicular to an optical axis of an optical system and the rotational axial direction of the deflector is referred to a main scanning direction below.
The subscanning direction in an optical system corresponds to a conveying direction of a transfer member in an image-forming apparatus while the main scanning direction corresponds to a direction perpendicular to the conveying direction in the plane of the transfer member. An image surface denotes a photosensitive drum surface, and an image plane represents a surface on which a beam is focused in practice.
In general, between an image processing speed (a paper conveying speed), an image resolution, a motor rotational speed, and the number of surfaces of a polygon mirror, the following relationship is found:                               P          *          R                =                              25.4            *            Vr            *            N                    60                                    (        1        )                                where        ,                                            P(mn/s): image processing speed (paper conveying speed)    R(dpi): image resolution (the number of dots per inch)    Vr(rpm): rotational speed of polygon motor    N: the number of surfaces of polygon mirror.
From the equation (1), the printing speed and the resolution are proportional to the number of surfaces of the polygon mirror and the rotational speed of polygon mirror. Hence, in order to achieve the speeding up and the resolution increasing, it is necessary to increase the number of surfaces of the polygon mirror or the rotational speed of polygon mirror.
However, in a conventional general under illumination scanning optical system, the width of a beam in the main scanning direction incident on the polygon mirror is smaller than that of the reflection surface of one mirror surface of the polygon mirror in the main scanning direction, so that the entire incident beam is reflected. The beam diameter on the image surface is proportional to an F number. An F number Fn is expressed by Fn=f/D, where a focal distance of an imaging optical system is f and the main scanning beam diameter on the polygon mirror surface is D. Accordingly, for increasing image quality, when the beam diameter on the image surface is to be reduced smaller, the main scanning beam diameter on the polygon mirror surface must be increased. Hence, when the number of surfaces of the polygon mirror is increased for achieving the speeding up and the resolution increasing, the polygon mirror must be increased in size. Then, when this is rotated at a high speed, the load to the motor increases. Hence, as electric power consumption of the motor increases or a large sized motor is used for corresponding to the load, cost is increased. Also, noise, vibration, and heat are largely produced, so that measures are required for them.
Then, as the measures therefor, an over illumination scanning optical system is effective. In the over illumination scanning optical system, the width of a beam in the main scanning direction incident on the polygon mirror is larger than that of one polygon mirror surface in the main scanning direction. Therefore, because the beam is reflected on the entire reflection surface, even when the number of reflection surfaces is increased as well as the beam diameter is secured on the polygon mirror for speeding up and increasing the resolution, the diameter of the polygon mirror can be reduced smaller.
Since the load to the polygon motor can be thereby reduced, the reduction in cost is possible. Also, because the polygon mirror is small in diameter and the number of surfaces is large, the shape of the polygon mirror approaches a circle, so that an air resistance is reduced, and the generation of noise, vibration, and heat can be reduced even when the polygon mirror is rotated at a high speed.
Due to the reduction in noise and vibration, a component such as glass may be eliminated or reduced, thereby also reducing the cost. A high duty cycle is also possible. With respect to this over illumination scanning optical system, there is a description in “Laser Scanning Notebook” (by Leo Beiser, SPIE OPTICAL ENGINEERING PRESS), for example.
However, by the above-mentioned conventional scanning optical system, unevenness in image density cannot be dissolved.
The transmission factor of a lens is maximal when a beam is perpendicularly incident on the lens, and is reduced with increasing angle to the normal of an incident surface (an incident angle). Hence, in the case of an fθ lens, the incident angle to the lens is increased at a scanning end, so that the transmission factor is smaller than that in the central scanning range, reducing the amount of light. Accordingly, the difference in amount of light between the central scanning range and the scanning end is large, and the image density difference is also increased.
On the countermeasure, therefore, the transfer factor is generally improved by the evaporation on the lens surface; however, when the fθ lens is made of plastics, if this lens surface is evaporated, the following problems arise.
When a plastic lens is used for the fθ lens, since it is exposed to elevated temperatures during the evaporation on the surface, the lens is deflected, deteriorating optical characteristics. In order to dissolve this problem, it is required to form the lens in view of the deflection. That is, it is necessary that the lens before the evaporation must be shaped to adjust it to the desired final shape after the deflection due to the evaporation. However, it is not easy to precisely analyze the deflection amount so that the manufacturing becomes very difficult. Furthermore, the evaporation increases the cost.
Moreover, in the case of the over illumination type, since the width of a beam varies with a scanning angle, unevenness in the amount of light on the image surface increases.
FIG. 1 shows widths of a beam reflected by the polygon mirror when the axis of the beam incident to the polygon mirror and an optical axis of an optical system after deflection are made to form an angle (in a case of not 0°).
If the reflection widths herein of the incident side to the polygon mirror (a), the central position of the scanning region (b), and the opposite side to the incidence (c) are D1, D2, and D3, respectively, the relationship is D1>D2>D3, so that dispersion is produced in the amount of light on the image surface.
FIG. 1 also shows intensity distributions of the beam before incidence to the polygon mirror. If the cross-sectional areas of the intensity distributions at positions (a), (b), and (c) are Sa, Sb, and Sc, respectively, the relationship is Sa>Sc and Sb>Sc because the beam width at each of the positions (a), (b), and (c) is different, and the used beam is a laser beam exhibiting the intensity distribution of Gaussian distribution. Since the amount of light is a volume of a used portion in the beam intensity distribution, if the amounts of light on image surfaces are Pa, Pb, and Pc, respectively, the relationship is Pa>Pc,>Pb>Pc. Hence, there is a problem that unevenness in the amount of light of the over illumination optical system is larger than that of the under illumination optical system, which is a conventional optical system.